Although wireless communication using electric waves plays a considerable role as an access line to a network like mobile communication or broadband communication, electric waves are a finite resource, and a frequency band is limited. On the other hand, with advancement of an information technology, a demand for high-speed/high-capacity communication is increased, and performing high-speed communication in a limited frequency band is desired. One of the solutions is application of a multi-level modulation scheme. Although the multi-level modulation is a scheme enabling transmission of an information volume including multiple bits by using one symbol, identifying symbols becomes difficult with realization of multiple values, and it is apt to undergo degradation in transmission characteristics due to noise, interferences, and non-linear characteristics of an amplifier.
FIG. 1 is a block diagram of 64-QAM as one of multi-level modulations using amplitude modulators. Information of six bits as transmission data is divided in parallel, modulators AM Mod-1 and AM Mod-2 convert two carrier waves perpendicular to each other into amplitude-modulated waves each having eight levels (corresponding to three bits), and respective outputs are subjected to vector combination by a combiner Σ. A combined 64-QAM signal wave is amplified by a power amplifier (HPA) and transmitted from an antenna ANT.
FIG. 2 shows a spatial signal arrangement view of a uniformly arranged 64-QAM signal wave. The 64-QAM signal wave has 10 values having different amplitudes and is affected by AM-AM and AM-PM conversions as non-linear characteristics of the power amplifier depicted in FIG. 3, and a spatial signal arrangement is transformed as shown in FIG. 4. Therefore, a reception side has a problem that an original signal can not be correctly demodulated and an error rate is increased.
To solve this problem, a transmission power must be increased by an amplifier having an excellent linearity, but there are drawbacks, i.e., a reduction in an efficiency of the amplifier in a region having the excellent linearity, a rise of price of the amplifier with an increase in the transmission power, a high capacity of a power supply, and an increase in price of the system due to, e.g., a countermeasure to heat generation.
When subjecting a multi-level modulation signal to power amplification, in order to suppress degradation in a transmission performance due to non-linear characteristics of the amplifier, an output is sufficiently lowered (output back-off) from a saturation point as shown in FIG. 3, and an operation is performed at an operation point where the linearity becomes excellent. However, this case has a drawback that an efficiency of the power amplifier is lowered. FIG. 5 shows a relationship between input-output characteristics and a power efficiency of a typical power amplifier. When an input power is increased, an output power is increased and a power efficiency also rises. The efficiency becomes maximum immediately before a region where the output demonstrates saturation. As apparent from this drawing, when output back-off is performed to put a high priority on the linear characteristics, the output power is reduced and the power efficiency is decreased at the same time. Therefore, to obtain a desired output, a power amplifier having a higher output is required, and there is a drawback of a high price of the system due to, e.g., an increase in a power consumption, a high capacity of a power supply, or a need for hardware for a measure against heat.
Meanwhile, as a conventional combining method for combining a plurality of signal waves having different types of information, there is one using a hybrid circuit, but it has a drawback of a reduction in a power combining efficiency due to a theoretical combination loss and a loss of a line of a circuit, and this is also a method using a plurality of antennas to individually combine a plurality of signal waves in a space, but it has a drawback that distances to reception points are different because reference points of the respective antennas are different, and gains (amplitude and phase) consequently become different from each other and a vector after combination varies in dependence on a reception position.
For example, FIG. 6 shows a method of combining two types of signal waves S-1 and S-2 by using a hybrid circuit (H), then amplifying the combined wave by a power amplifier PA, and transmitting this wave from one antenna ANT, and FIG. 7 shows a method of individually amplifying two types of signal waves S-1 and S-2 by power amplifiers PA-1 and PA-2, then combining these waves by a hybrid circuit H, and transmitting this wave from an antenna ANT, but a combination loss of 3 dB theoretically occurs in the hybrid circuit H in this case. Further, a loss due to a circuit loss also occurs.
Furthermore, FIG. 8 shows a method of combining four types of signal waves S-1, S-2, S-3, and S-4 by using hybrid circuits H-1, H-2, and H-3 and then transmitting the combined wave from one antenna ANT. In this case, a combination loss of 3 dB likewise occurs in each of the hybrid circuits H-1 to H-3. Moreover, a loss due to a circuit loss also occurs. Any other types of signal waves can be combined in the same manner. FIG. 9 shows a method of combining two types of signal waves S-1 and S-2 in a space by using two antennas ANT-1 and ANT-2. In this case, phase shifters φ-1 and φ-2 are adjusted so as to have the same phase in a desired direction, and then power amplifiers PA-1 and PA-2 individually perform amplification. Although two or more types of signal waves can be likewise combined by using a plurality of antennas, in the technology depicted in FIG. 6, the power amplifiers PA has to be operated in regions having an excellent linearity in order to avoid degradation in transmission characteristics due to an influence of non-linear characteristics of the power amplifiers PA, and hence there is a drawback of a reduction in a power efficiency.
Additionally, the conventional combining method using the hybrid circuits depicted in FIG. 7 or FIG. 8 has the drawbacks that the combination loss of 3 dB theoretically occurs in each hybrid circuit, the combination loss is large, and the combination efficiency is decreased as explained above.
Further, in the conventional combination method using the plurality of antennas depicted in FIG. 9, assuming that a distance between the two antennas is d, a path difference d·sin θ occurs in lengths of propagation paths to reception points in a service area SA in infinity along the θ direction as shown in FIG. 10. As a result, phases of both signal waves at the reception points in the service area SA are different from each other except for a case where an angle θ=0 as depicted in FIG. 11. That is, a vector after combination differs depending on each reception point since both the phases are different depending on the angle θ.
Patent Document 1: Japanese Patent Application Laid-open No. 1996-288970
Patent Document 2: Japanese Patent Application Laid-open No. 1997-238171